DESCRIPTION: Smooth muscle (SM) is a major cell type in many of the body's systems (vascular, digestive, reproductive, etc) whose abnormal function is a major cause of morbidity and mortality in the United States. The objective of this project is to investigate the functional role of the unique isoforms of the contractile protein myosin present in smooth muscle cells (SMCs). Both the SM myosin heavy chain (MHC) head and tail isoforms and the myosin light chain (MLC17) isoforms result from alternative splicing. The expression of these isoforms varies between species and tissue types and is also developmentally regulated. In addition, the expression of these isoforms is affected in variou diseased states (atherosclerosis, etc). While there is good evidence that skeletal and cardiac muscle myosin isoforms correlate with function in these tissues, no such structure/function relationship has been shown for SM myosin. In contrast, there are reports that changes in the relative amounts of specifi SM myosin isoforms (heavy and light chains) do not correlate, cause an increas or a decrease in shortening velocity. These variable results may be due to the extreme intercellular variability of SMCs with regard to their myosin isoform content as indicated by both immunohistochemical studies and molecular studies at the single cell level. This project is designed to address the hypotheses that the: (1) SM MHC isoforms are a determinant of SMC shortening velocity, (2 SM MHC tail isoforms are a determinant of the extent of SMC shortening, and (3 SM MLC17 isoforms modulate shortening velocity, but not the extent, of shortening. Methods have been designed to allow mechanical measurements and relative quantitation of the SM myosin isoforms at the single cell level. Thes include the use of a very sensitive force transducer and "motor" and reverse transcription polymerase chain reaction. Importantly, both the mechanical measurements and the relative quantitation are done on the same single cell. Thus, these studies will not be affected by intercellular variability. This project is designed to address the regulation and function of the SM MHC and MLC isoforms by working at the single cell level, where many of the confoundin factors present in multicellular preparations are not a problem. These results will advance our understanding of structure/function relationships for these S myosin isoforms.